The present invention relates to a toner for use in electrophotography, an image forming method for visualizing an electrostatic image and toner jetting; an image forming method using the toner, and a process cartridge including the toner.
Hitherto, various electrophotographic processes have been disclosed in U.S. Pat. Nos. 2,297,691; 3,666,363: 4,071,361; etc. In these processes, an electrostatic latent image is formed on a photoconductive layer by irradiating a light image corresponding to an original, and a toner is attached onto the latent image to develop the electrostatic image. Subsequently, the resultant toner image is transferred onto a transfer(-receiving) material such as paper, via or without via an intermediate transfer member, and then fixed, e.g., by heating, pressing, or heating and pressing, to obtain a copy or a print. The toner remaining on the photosensitive member is cleaned by various methods, and the above steps are repeated for a subsequent image forming cycle.
Japanese Laid-Open Patent Application (JP-A) 55-18656 has proposed a jumping developing method wherein a magnetic toner is applied in a very small thickness onto a sleeve, triboelectrically charged and brought to a proximity to an electrostatic image to effect the development. This method is advantageous in that a sufficient triboelectrification becomes possible by application of the magnetic toner in a very small thickness layer on the sleeve to increase the opportunity of contact between the sleeve and the toner.
However, the developing method using an insulating magnetic toner involves an unstable factor associated with the use of such an insulating magnetic toner. More specifically, insulating magnetic toner particles contain a substantial amount of fine powdery magnetic material, and a portion of the magnetic material is isolated from or exposed to the surfaces of the toner particles, thus affecting the flowability and triboelectric chargeability of the magnetic toner to consequently change or deteriorate the various performances, inclusive of developing performance and continuous image forming performances. These difficulties are presumably caused by the presence at the magnetic toner particle surfaces of fine particles of magnetic material having a lower resistivity than the resin constituting the toner. The toner chargeability also greatly affects the developing performance and transferability, thus also deeply affecting the resultant image quality. For this reason, a magnetic toner capable of stably attaining a high charge is seriously demanded.
Further, in recent years, apparatus utilizing electrophotography have been used not only as copying machines for reproducing originals but also for printers for computers and facsimile apparatus. Accordingly, electrophotographic apparatus are required to be smaller in size and weight and to exhibit higher speed and reliability, so that they are required to be composed of simpler components. Consequently, a toner is required to exhibit higher performances, failure of which makes impossible the realization of an excellent image forming apparatus.
JP-A 7-230182 and JP-A 8-286421 have proposed external addition of magnetic material powder for stabilizing the chargeability. This allows the provision of a toner showing a stable chargeability and high cleanability, but the toner is liable to be attached to a contact charging member which is frequently included in a high-speed printer of a simple structure.
Further, after a transfer step of transferring a toner image from a photosensitive member to a transfer(-receiving) material, a portion of toner (residual toner) remains on the photosensitive member without being transferred. The residual toner has to be cleaned from the photosensitive member in order to continuously obtain good toner images in a continuous copying or printing. The recovered residual toner is stored in a vessel in the image forming machine or a recovery box and then discharged as a waste toner or recycled.
In order to obviate the occurrence of waste toner, the image forming apparatus has to be equipped with a recycle mechanism. Such a recycle system to be placed in the apparatus has to be a large-scale one for complying with multiplicity of function, high-speed and high image quality required of copying machines, printers and facsimile apparatus demanded on the market, thus resulting in a larger apparatus which is against the demand for a smaller apparatus in the market. This problem is also encountered also in the case of storing the waste toner in a vessel or a recovery box disposed in the apparatus or in a system including a waste toner recovery unit integral with the photosensitive member.
In order to alleviate the problem, the rate or efficiency of transfer at the time of transferring a toner image from a photosensitive member to a transfer material has to be increased.
JP-A 9-26672 has proposed a toner containing a transfer efficiency-improving agent having an average particle size of 0.1-3 xcexcm and hydrophobic silica fine powder having a BET specific surface area of 50-300 m2/g, so that the toner is provided with a reduced volume resistivity and a thin layer of the transfer efficiency-improving agent is formed on the photosensitive member, to increases the transfer efficiency. However, a toner produced through the pulverization process is caused to have a generally broad particle size distribution, so that it is difficult to uniformly increase the transfer efficiency of all the toner particles, thus leaving a room for further improvement.
For improving the transfer efficiency, there has been known a method of forming a toner, of which the shape is made closer to a sphere. Examples thereof may include production methods by spraying toner particle formation, dissolution with a solution and polymerization as disclosed in JP-A 3-84558, JP-A 3-229268, JP-A 4-1766 and JP-A 4-102862. However, these toner production methods require a large production apparatus, and the resultant sphere-like toner particles are liable to cause a problem of cleaning failure because of their spherical shape.
In a conventional toner production process including a pulverization step, toner ingredients including a binder resin for ensuring toner fixation onto a transfer material, a colorant or magnetic material for providing a toner and a charge control agent for imparting a chargeability to toner particles are dry-blended and melt-kneaded by a kneading apparatus, such as a roll mill or an extruder, and, after being cooled and solidified, the kneaded product is pulverized by a pulverizing apparatus, such as a jet stream-type pulverizer or a mechanical impingement-type pulverizer, followed by classification by means of a pneumatic classifier, to obtain toner particles, which are optionally further blended with a flowability improver and a lubricant externally added thereto. In order to provide a two-component developer, the toner may be blended with a magnetic carrier.
An example of such a process for producing toner particles is illustrated by a flow chart shown in FIG. 7.
A coarsely pulverized material is continuously or successively fed to a first classification means, from which a coarse powder fraction principally comprising particles beyond a prescribed particle size range is sent to a pulverization means for pulverization and then recycled to the first classification means.
The other fine powder fraction principal comprising particles within the prescribed particle size range and particles below the prescribed particle size range is supplied to a second classification means and separated thereby into medium powder principally comprising particles within the prescribed particle size range, fine powder principally comprising particles below the prescribed particle size range and coarse powder principally comprising particles above the prescribed particle size range.
As the pulverization means, various pulverizers are used, and for pulverization of a coarsely pulverized toner product principally comprising a binder resin, an impingement-type pneumatic pulverizer using a jet gas stream as shown in FIG. 9 is generally used.
In such an impingement-type pneumatic pulverizer using a high pressure gas for a jet gas stream, a powdery material is conveyed with a jet air stream and ejected from an outlet of an acceleration pipe to be impinged onto an impingement surface of an impingement member disposed opposite to the outlet opening of the acceleration pipe, whereby the powdery material is pulverized by an impact force caused by the impingement.
For example, in the impingement-type pneumatic pulverizer shown in FIG. 9, an impingement member 164 is disposed opposite to an outlet port 163 of an acceleration pipe 162 connected to a high-pressure gas feed nozzle 161, a powdery material is sucked through a powder material feed port 165 formed intermediate the acceleration tube 162 into the acceleration tube 162 under the action of a high-pressure gas supplied to the acceleration pipe, and the powder material is ejected from the outlet port 163 together with the high-pressure gas to impinge onto the impinging surface 166 of the impingement member 164 to be pulverized under the impact. The pulverized product is discharged out of a discharge port 167.
However, as the powdery material is pulverized by the impacting force caused by the impingement of the powder ejected together with a high-pressure gas onto the impingement member, the resultant toner particles are made indefinitely shaped and angular, and the release agent and magnetic material powder are liable to be isolated from the toner particles.
JP-A 2-87157 discloses a method wherein toner particles produced through the pulverization process are subjected to a mechanical impact (by means of a hybridizer) to modify the shape and surface state of the particles to improve the transfer efficiency. According to this method, however, as a treatment step is added after the pulverization process, the productivity of toner particles is lowered and toner particle surface is made less uneven to require some improvement in developing performance.
Further, in order to produce a small particle size toner by using the above-mentioned impingement-type pneumatic pulverizer, a large amount of air is required, thus increasing the electric power consumption which results in an increase in production energy cost. In recent years, economization of toner production energy is also required from an ecological viewpoint.
As for the classification means, various pneumatic classifiers and classifying methods have been proposed, including classifiers using rotating vanes and classifiers having no moving units. The latter includes a fixed wall-type centrifugal classifier, and a classifier utilizing an inertia. The use of the latter inertia-type classifiers has been proposed in Japanese Patent Publication (JP-B) 54-24745, JP-B 55-6433 and JP-A 63-101858.
According to such a pneumatic classifier, as illustrated in FIG. 10, a powdery material is ejected together with a high-speed gas stream through a supply nozzle opening into a classification zone of a classification chamber, and under the action of a centrifugal force caused by a curved gas stream flowing along a Coanda block 145, the powdery material is classified into coarse powder, medium powder and fine powder which are separated by narrow-tipped edges 146 and 147.
More specifically, in a classification apparatus 127, a pulverized powder material is introduced through a supply nozzle including tapered tubular pipe suctions 148 and 149, where a powdery material tends to flow straightly and parallel to the tube walls. However, in the supply nozzle, the powder supply stream is liable to be separated into an upper stream rich in light fine powder and a lower stream rich in heavier coarse powder. The respective powder streams are liable to flow separately and be ejected in different courses depending on positions of introduction into the classifying chamber, and further the coarse powder stream is liable to disturb the course of flying of fine powder, thus posing a limit of improved classification accuracy.
Moreover, a large number of different properties are required of a toner, and many of them are determined not only by the starting materials but also by the production processes. The toner classification step is required to provide classified particles having a sharp particle size distribution at a low cost and in a stable manner.
Further, in recent years, toner particles are gradually becoming smaller in size in order to improve the image quality in copying machines and printers in recent years. Generally, a particulate substance is governed by a larger inter-particle force as the particle size becomes smaller. This is also true with toner particles principally comprising a resin, and the agglomeratability thereof becomes larger as the size thereof is smaller.
As a result, in the case of obtaining a toner having a weight-average particle size of at most 10 xcexcm and a sharp particle size distribution, the classification efficiency is significantly lowered by using conventional apparatus and methods. Particularly in the case of obtaining a toner having a weight-average particle size of at most 10 xcexcm and a sharp particle size distribution, not only the classification efficiency is significantly lowered, but also the classified toner particles are liable to have a large amount of an ultra-fine powder fraction, by using conventional apparatus and methods.
Further, according to the conventional system, even if a toner product having an accurate particle size distribution can be attained, the steps therein are liable to be complicated to result in a lower classification efficiency, a lower production yield and a higher production cost. This tendency becomes more noticeable if the prescribed size becomes smaller.
Further, in the case of a magnetic toner having a smaller particle size than usual, the amount of magnetic material contained in toner particles is increased in order to suppress the fog, and the amount of magnetic material isolated from the toner particle is increased correspondingly. As a result, in order to comply with a higher process speed, the lowering in low-temperature fixability and restriction on developing performance of a magnetic toner become severer than ever.
A generic object of the present invention is to provide a dry magnetic toner having solved the above-mentioned problems.
A more specific object of the present invention is to provide a dry magnetic toner capable of retaining a good developing performance even at a smaller particle size.
Another object of the present invention is to provide a dry magnetic toner causing less waste toner to exhibit a higher transfer rate.
A further object of the present invention is to provide a process cartridge and an image forming method using such a magnetic toner.
According to the present invention, there is provided a dry magnetic toner, comprising: magnetic toner particles each comprising at least a binder resin and magnetic iron oxide particles; wherein
100-350 iron-containing isolated particles are present per 10,000 toner particles;
the toner has a weight-average particle size X in a range of 5-12 xcexcm; and contain at least 90% by number of particles satisfying a circularity Ci according to formula (1) below of 0.900 with respect to particles of 3 xcexcm or larger therein,
Ci=L0/Lxe2x80x83xe2x80x83(1),
xe2x80x83wherein L denotes a peripheral length of a projection image of an individual particle, and L0 denotes a peripheral length of a circle giving an identical area as the projection image; and
the toner satisfies either
(a) (i) a cut percentage Z determined by formula (3) shown below satisfies formula (2) below with respect to the weight-average particle size X:
Zxe2x89xa65.3xc3x97Xxe2x80x83xe2x80x83(2),
Z=(1xe2x88x92B/A)xc3x97100xe2x80x83xe2x80x83(3),
xe2x80x83wherein A denotes the number of total particles and B denotes the number of particles of 3 xcexcm or larger, and
(ii) the toner contains a number-basis percentage Y (%) of particles having Cixe2x89xa70.950 within particles of 3 xcexcm or larger satisfying:
Yxe2x89xa7Xxe2x88x920.645xc3x97exp5.51xe2x80x83xe2x80x83(4), or
(b) (iii) a cut percentage Z determined by the formula (3) above satisfies formula (5) below with respect to the weight-average particle size X:
Z greater than 5.3xc3x97Xxe2x80x83xe2x80x83(5), and
xe2x80x83percentage Y (%) of particles having Cixe2x89xa70.950 within particles of 3 xcexcm or larger satisfying:
Yxe2x89xa7Xxe2x88x920.545xc3x97exp5.37xe2x80x83xe2x80x83(6).
According to another aspect of the present invention, there is provided an image forming method, comprising the steps of:
developing an electrostatic image formed on an image-bearing member with the above-mentioned dry magnetic toner to form a toner image thereon,
transferring the toner image onto a transfer material via or without via an intermediate transfer member, and
fixing the toner image onto the transfer material under application of heat and pressure.
According to a further aspect of the present invention, there is provided a process-cartridge comprising: an image-bearing member, and a developing means containing the above-mentioned dry magnetic toner for developing an electrostatic image formed on the image-bearing member; the image-bearing member and the developing means being integrally supported to form a cartridge which is detachably mountable to a main assembly of image forming apparatus.